The nocturnal sleep patterns of 10 patients with myasthenia gravis and five controls were recorded in the conventional manner for 7 hours on two consecutive nights. One patient was retested 4 weeks after institution of prednisone therapy. All the myasthenics had a significant disturbance in rapid eye movement (REM) sleep cycles. In the patient who was retested after clinically successful prednisone therapy, the REM sleep pattern had become normal. Since acetylcholine is the putative brain stem transmitter substance involved in the maintenance of REM sleep, our findings suggest a disturbed central mechanism of acetylcholine in myasthenia gravis.